An avalanche photodiode (APD) is a type of photosensitive semiconductor device in which light is converted to electricity due to the photoelectric effect coupled with electric current multiplication as a result of avalanche breakdown. APDs differ from conventional photodiodes in that incoming photons internally trigger a charge avalanche in APDs, thus APDs can measure light of even lower level and are widely used in long-distance optical communications and optical distance measurement where high sensitivity is needed.
Germanium/Silicon (GeSi) APDs combine the characteristic of excellent optical absorption of Ge at telecommunication wavelength with the characteristic of outstanding carrier multiplication properties of Si. The use of Ge allows the extension of the spectral response of GeSi APDs to longer wavelengths, up to 1550 nm. However, the absorption of bulk Ge ceases at 1550 nm at room temperature, which is limited by its bandgap in Gamma band. Since there is a requirement for the optical band in optical communication systems to cover a wide wavelength range, from 1260 nm to 1620 nm, the longer wavelength limitation of optical absorption of Ge is a main reason restricting the wide application of GeSi APDs in optical communication systems. Therefore, there is a need to extend the absorption of Ge to longer wavelengths above 1550 nm.
One of the parameters that impact the applicability and usefulness of APDs is dark current. Dark current is a relatively small electric current that flows through a photosensitive device, such as a photodiode, even when no photons are entering the photosensitive device. Dark current is one of the major sources of noise in photosensitive devices. Consequently, dark current is a limiting factor for GeSi APDs in high-speed optical communication applications. Therefore, there is a need to reduce the dark current to achieve high performance in APDs.